AP BiologyChapter 13:Gene Regulation

Gene Regulation in Bacteria and Eukaryotes

Bacterial cells• Genetic Organization?• Grow rapidly and have short life span• Controlling transcription is the most economical way for the

cell to regulate gene expression

Eukaryote cells• Long life span/respond to many different stimuli• A single gene is regulated in different ways in different types

of cells• Gene regulation complex• Although transcriptional-level control predominates, control

at other levels of gene expression is also important

Gene Regulation in Bacteria Prokaryotic DNA is organized into units called operons,

which contain functionally related genes Operons regulated as units, so functionally related proteins

are synthesized simultaneously only when needed Each operon consists of:

Regulatory gene, controls transcription of other genes Promoter, RNA polymerase recognizes as place to start transcribing Operator, governs access of RNA polymerase to promoter

Structural genes, encode for related proteins

Inducible, Repressible, and Constitutive Genes in Bacteria

Inducible operon• Normally turned off• Catabolic pathways

Repressible operon• Normally turned on• Operate via feedback inhibition• Anabolic pathways

Constitutive genes• Constantly needed and therefore constantly transcribed• Examples: Ribosomal proteins, tRNAs, RNA polymerase, glycolysis

enzymes• Neither inducible nor repressible and active at all times• The activity of constitutive genes is controlled by how efficiently RNA

polymerase binds to their promoter regions.

Inducible Operons: lac operon Intestinal bacterium Escherichia coli (E.coli) lives on what its

host eats Specific enzymes are needed to metabolize the type of food

that comes along e.g. in newborn mammals, E.coli are bathed in milk,

containing the milk sugar lactose The lactose operon contains three structural genes, each

coding for an enzyme that aids in lactose metabolism lactose not present: repressor active, operon off; no

transcription for lactose enzymes lactose present: repressor inactive, operon on; inducer

molecule inactivates protein repressor (allolactose) transcription is stimulated when inducer binds to a

regulatory protein Lac Operon Video

Lac Operon Video

Allolactose bound to repressor

Repressible Operons:trp Operon

Tryptophan synthesis (anabolism) Promoter: RNA polymerase binding

site; begins transcription Operator: controls access of RNA

polymerase to genes (tryptophan not present)

Repressor: binds to operator preventing attachment of RNA polymerase ~ when tryptophan is present ~ acts as a co-repressor)

Transcription is repressed when tryptophan binds to a regulatory protein

Trp Operon Video

Negative Control in the Regulation of an Operon

Negative regulators inhibit transcriptionRepressible and inducible operons are under

negative controlWhen repressor protein binds to operator,

transcription is turned offSeen in lac and trp operons

Positive regulators stimulate transcription Some inducible operons (lac) are also under

positive controlA separate protein binds to DNA and stimulates

transcription of the genePositive control of lac operon requires that the cell

is able to sense the presence of glucose• More efficient for cell to utilize glucose before lactose• Only when lactose is present and glucose is in short

supply does E.coli use lactose as energy source

Positive Control in the Regulation of an Operon

Positive Regulation of lac Operon

Lac operon always has low affinity for RNA polymerase Involves Two Proteins:

• CAP (catabolite activator protein) • cAMP (cyclic AMP)

Together, CAP and cAMP cause RNA polymerase to bind tightly to promoter region Levels of cAMP increase as levels of glucose decrease Lac operon is fully active only when lactose is available and glucose levels are low

A Regulon Group of functionally related operons controlled

by a common regulator Example: CAP regulates the catabolism of

lactose, galactose, arabinose and maltose

Comprehension CheckMatch these components of the lac operon with their functions.

______ b-galactosidase A. is inactivated when attached to

lactose______ cAMP-CAP complex B. codes for synthesis of

repressor______ lactose C. hydrolyzes lactose______ operator D. stimulates gene

expression______ promoter E. repressor attaches here______ regulator gene F. RNA polymerase attaches

here______ repressor G. acts as inducer that

inactivates repressor______ structural gene H. codes for an enzyme

C

D

G

E

F

B

A

H

Comprehension CheckListed below are characteristics of repressible

and inducible enzymes. Identify each of the following as true of repressible or inducible enzymes.

______ genes are switched off until a specific metabolite inactivates the repressor

______ genes are switched on until a specific metabolite activates the repressor

______ Generally function in anabolic pathways______ Usually function in catabolic pathways______ Pathway end product switches off its own

production______ Enzyme synthesis is switched on by the

nutrient in used in the pathway

Inducible

Repressible

RepressibleInducibleRepressible

Inducible

Comprehension CheckThe events listed below describe how the lac operon functions

when lactose is present and glucose is absent. Put the steps in the correct order.______ Allolactose binds to repressor______ cAMP accumulates______ cAMP activates CAP______ cAMP binds to CAP______ cAMP/CAP complex binds to CAP site in

promoter______ CAP concentration increases______ Genes transcribed______ Repressor inactivated______ RNA polymerase binds to promoter

1

2

3

4

5

6

7

8

9

Eukaryotic Gene Expression

Not organized into operons Typical human cell only expresses about 20%

of its genes at any given time Remember: All body cells contain identical

genome Cells rely on differential gene expression

Regulation allows cell differentiation and organization into tissues/organs

Each gene has regulatory sequences essential to the control of transcription

Gene Regulation in Eukaryotic Cells

Gene regulation occurs at the levels of

• Transcription

• mRNA processing

• Translation

• The protein product

Eukaryotic Promoters Vary in Efficiency, Depending on UPE’s

Like prokaryotes, transcription requires an initiation and promoter sites

Eukaryotic Promoter consists of:• RNA Polymerase-binding Site (TATA box)• Upstream Promoter Elements (UPE’s)

• 8-12 bases upstream from TATA box

Types/Number of UPE’s determine efficiency of promoter

• UPE’s required for accurate and efficient initiation of transcription

In addition to UPE’s, eukaryotic genes also controlled by Enhancers

• Enhancers facilitate RNA polymerase binding to promoter

• Increase rate of transcription

Regulationof Transcriptionin Eukaryotes

Eukaryotic Regulatory Proteins

Called “Transcription Factors”

Similar to repressors and CAP’s in prokaryotes

Usually act as activators

Enhancers only become functional when bound to specific transcription factors

Chromosome Organization may Affect Gene Expression

Genes are inactivated by changes in chromosome structure• Heterochromatin is tightly wound and not transcribed (ex. Barr body)• Euchromatin is loosely packed and easily transcribed

DNA methylation• Methyl groups added to cytosines• Make transcription impossible

Multiple copies of some genes present in one chromosome• Tandemly Repeated Gene Sequences (VTNR’s)

Gene amplification• Cells produce multiple copies of a gene by selective replication

Differential mRNA Processing(Posttranscriptional Control)

Cells in each tissue produce own version of mRNA Same gene can be used to produce a certain protein in one tissue and a

related, but slightly different protein in another tissue Example: troponin

• a protein that regulates muscle contraction• produced in different muscle tissues

Other Methods of Posttranscriptional Control

Proteolytic Protein ProcessingProteins produced in inactive formBecome active via removal of a portion of their

polyepeptide chain Chemical Modification

Addition or removal of functional groupsAffects enzyme activityKinases (add phosphate groups)Phosphatases (remove phosphate groups)